The present invention relates to a polycrystal thin film forming method and forming system, more specifically a polycrystal thin film forming method and forming system for forming at low temperature polycrystal thin film on a substrate of low heat resistance temperature.
Furthermore, the present invention relates to a semiconductor thin film forming method, a thin film transistor fabrication method and a liquid crystal display device fabrication method, more specifically a semiconductor thin film forming method which can form semiconductor thin film having good crystallization on a substrate having low heat resistance temperature, a thin film transistor fabrication method for fabricating a thin film transistor using the semiconductor thin film, and a liquid crystal display device for fabricating a liquid crystal display device using the thin film transistor.
Recently, liquid crystal displays (LCDs) using thin film transistors (TFTs) as switch devices for the picture elements because of electric power saving, space saving, high response speed, beautiful display, etc.
Such liquid crystal displays generally use glass substrates, and the thin film transistors are formed on the glass substrate. The channel layers of the thin film transistors are formed of, in many cases, polycrystal silicon thin film.
As a method for forming polycrystal silicon thin film on a glass substrate has been conventionally known a method in which amorphous silicon thin film is formed on a glass substrate and then is subjected to a heat treatment at 600xc2x0 C. for 50 hours to crystallize the amorphous silicon thin film, and polycrystal silicon thin film is formed. In this method nuclei of crystals are grown at the initial stage of the heat treatment, and the nuclei are grown to form polycrystal silicon thin film.
However, in this polycrystal silicon thin film forming method the heat treatment performed at 600xc2x0 C. for about 50 hours deforms the glass substrate. Furthermore, crystal grain of the thus-formed polycrystal silicon thin film have many defects and twins. Thus this method has found it difficult to form high-quality polycrystal silicon thin film having high electron mobility.
It was considered to form polycrystal silicon thin film on a glass substrate at an above 600xc2x0 C. high temperature by CVD (Chemical Vapor Deposition), but the glass substrate was deformed by the high temperature of above 600xc2x0 C., and the thus-formed polycrystal silicon thin film could not have sufficient crystallization.
Then is proposed a method in which amorphous silicon thin film is formed on a glass substrate, and laser beams are applied to the amorphous silicon thin film to form polycrystal silicon thin film. In this method polycrystal silicon thin film is formed in the process of the silicon melted by the laser beams solidifying. The amorphous silicon thin film is melted by the laser beams for a short period of time without heating the glass substrate to a high temperature. Accordingly polycrystal silicon thin film can be formed without deforming the glass substrate.
However, in this proposed polycrystal silicon thin film forming method because silicon solidifies at high speed, polycrystal silicon thin film having large crystal grain diameters cannot be formed. Thin film transistors using the thus-formed polycrystal silicon thin film as the channel layers have electron mobilities so low as about 150 cm2/Vs.
An object of the present invention is to provide a method for forming a semiconductor thin film which can provide high electron mobility even when the film is formed at low temperature, a thin film transistor using the semiconductor thin film, and a liquid crystal display device using the thin film transistor.
The above-described object is achieved by a polycrystal silicon thin film forming method comprising the steps of: forming a silicon layer on a substrate; forming a heat reservoir layer on an upper surface of the silicon layer and side surfaces of the silicon layer; and applying the short pulsed laser beams to the silicon layer to crystallize the silicon layer. Because of the heat insulation layer covering the silicon layer, the silicon layer after subjected to laser beam application can have low cooling speed, whereby a polycrystal silicon thin film having large grain diameters can be formed. Accordingly, a polycrystal silicon thin film having high electron mobility can be formed.
The above-described object is achieved by a semiconductor device including an active semiconductor film formed on an insulating substrate, at least a channel region of the active semiconductor film having a quasi-monocrystal state, which is a crystal state containing only grain boundaries having inclination angles of not more than 90 to a current direction.
The above-described object is achieved by a semiconductor device including an active semiconductor film formed on an insulating substrate, at least a channel region of the active semiconductor film having a polycrystal state which is formed of circular large-diameter crystal grain, a radius L of the circular large-diameter crystal grain being larger than 250 nm, and the radius L being larger than W/4 when a width of the channel is represented by W.
The above-described object is achieved by a method for fabricating a semiconductor device including an active semiconductor film formed on an insulating substrate, comprising the steps of: forming a semiconductor film in the shape of an island on one surface of the insulating substrate; covering the semiconductor film with an isolation film, covering a side of the semiconductor film with a heat retaining film through the isolation film; and crystallizing the semiconductor film by applying energy beams to the semiconductor film from said one surface of the insulating substrate to form the active semiconductor film.
The above-described object is achieved by a method for fabricating a semiconductor device including an active semiconductor film formed on an insulating substrate, comprising the steps of: forming a semiconductor film in the shape of an island on one surface of the insulating substrate; covering the semiconductor film with an isolation film, covering the entire surface of the semiconductor film with the heat retaining film through the isolation film; and crystallizing the semiconductor film by applying energy beams to the semiconductor film from the other surface of the insulating substrate to form the active semiconductor film.
The above-described object is achieved by a method for forming a silicon thin film, comprising the steps of: forming a silicon layer on one surface of an insulating substrate; forming a heat retaining layer on at least a side of the silicon layer; and applying continuous-wave energy beams to the silicon layer to crystallize the silicon layer.